Colloidal rare earth elements in a boreal river: Changing sources and distributions during the spring flood

Variations in the physico-chemical speciation of the rare earth elements (REE) have been investigated in a subarctic boreal river during an intense spring flood event using prefiltered (<100 μm) samples, cross-flow (ultra)filtration (CFF), flow field-flow fractionation (FlFFF), and diffusive gradients in thin films (DGT). This combination of techniques has provided new information regarding the release and transport of the REE in river water. The colloidal material can be described in terms of two fractions dominated by carbon and iron, respectively. These two fractions, termed colloidal carrier phases, showed significant temporal changes in concentration and size distribution. Before the spring flood, colloidal carbon concentrations were low, the colloids being dominated by relatively large iron colloids. Colloidal concentrations increased sharply during the spring flood, with smaller carbon colloids dominating. Following the spring flood, colloidal concentrations decreased again, smaller carbon colloids still dominating. The REE are transported mainly in the particulate and colloidal phases. Before the spring flood, the REE composition of all measured fractions was similar to local till. During the spring flood, the REE concentrations in the colloidal and particulate fractions increased. The increase was most marked for the lighter REE, which therefore showed a strong enrichment when normalized to local till. Following the spring flood, the REE concentrations decreased again and reverted to a distribution similar to local till. These changes in the concentration and distributions of carbon iron and REE are interpreted in terms of changing hydrological flow paths in soil and bedrock which occur during the spring flood.     

Flood zone biogeochemistry of the Ob River middle course

The flood zone of the Ob River, the largest (in watershed area) river of the Arctic Ocean basin, is tens of km wide and, after the Amazon's Varzea, is the world's second largest flooding territory. To better understand the biogeochemistry of the Ob River and adjacent surface waters, we studied, in May and July 2014, the dissolved and colloidal organic carbon and trace metals in small rivers, lakes, and flooded water bodies connected and disconnected with the mainstream as well as the Ob River itself. All major and trace elements were distributed among two major categories depending on their pattern of dependence on the dissolved organic carbon (DOC) concentration. Dissolved inorganic carbon (DIC), Na, Mg, Ca, sulfate, Sr, Mo, Sb and U exhibited a general decrease in concentration with the increase of the [DOC]. The lowest concentration of these elements was observed in DOC-rich humic, acidic (4.9 ≤ pH ≤ 6.1) upland lakes fed by surrounding bogs. These elements marked the influence of underground feeding in July during summer baseflow, which was most visible in flood lakes in the Ob riparian zone and the Ob River itself. In May, the flood lakes were statistically similar to the Ob River. The elevated concentration of DOC (up to 60 mg/L) in the upland lakes was not correlated with groundwater-related elements, suggesting a lack of significant groundwater feeding in these lakes. In contrast, insoluble, usually low mobile elements (Al, Fe, other trivalent hydrolysates, Ti, Zr, Hf) and some metals (Cr, Zn, Ni, Pb) demonstrated a steady increase in concentration with increasing DOC, with the lowest values observed in the Ob River and the highest values observed in small tributaries and organic-rich upland lakes in July. It follows that these elements are limited by their main carriers – organic and organo-ferric colloids, rather than by the availability of the source, peat and mineral soil or plant litter. While for the majority of non-colloidal, groundwater-fed elements with high mobility (DIC, Na, Mg, Ca, K, Sr…) the small tributaries can be used as representatives of the Ob main stream, this is not the case for low mobility “insoluble” elements, such as Fe, Al, trivalent and tetravalent hydrolysates, and metal micronutrients (Cu, Zn, and Mn). The low soluble elements and divalent metals exhibited a much lower concentration in the river mainstream compared to that in the flood lakes, upland lakes and small rivers. This difference is significantly more pronounced in the baseflow in July compared to the spring flood in May. Presumably, autochthonous processes, such as the photo-oxidation and bio-oxidation of organo-ferric colloids and phytoplankton uptake are capable decreasing the concentration of these elements in the river mainstream.     

中国东部主要入海河流河口区地球化学特征：理化指标与水溶态元素浓度

河水理化性质和元素组成特征直接影响到水体利用功能,入海河流元素输送量对近岸海洋生态环境具有重要影响。在中国东部33条入海河流下游河段或河口区布设了水地球化学调查点,分别在2007年夏季(丰水期)、2007年底—2008年初(枯水期)采样并测定了河水酸碱度、电导率值以及溶解态常量和微量元素浓度。研究表明,多数北方河流水体酸碱度、电导率值以及常量元素浓度高于南方河流,与中国土壤及其常量元素组成的南北分带相吻合,反映了我国南北气候分带对河水地球化学特征的控制作用,推断少数河流酸碱度、电导率和常量元素浓度的异常分布与海水混合作用、人为污染等作用有关;部分河流水体中微量元素浓度背离于区域正常浓度,其原因一是与区域地球化学背景有关,二是与城市污染影响有关,通过对比部分城市上、下游水体元素浓度证实城市污染的影响;研究认为丰水期易溶元素Na、Ca、K、Zn、Se浓度较低反映了大量降水的稀释效应,而丰水期河水Al、REE、Fe、Pb、Tl等元素浓度明显高于枯水期,主要与丰水期暴雨形成的地面径流携带大量胶体颗粒进入地表水有关。     

Dissolved osmium in Bengal plain groundwater: Implications for the marine Os budget

We report osmium concentrations and isotopic compositions of 40 groundwater samples from the Bengal plain. Groundwaters have Os concentrations (16.9-191.5 pg/kg), about 5-10 times higher than those published for most rivers or seawater. ¹⁸⁷Os/¹⁸⁸Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers. Os contents are correlated with those of soluble elements such as Sr, Mg, and Ca, suggesting that differing extents of solid-solution interaction explain most of the variation in measured Os concentrations. The covariation between Os and Sr allows us to estimate the mean Os content of Bengal groundwater (∼70 pg/kg). This concentration is too low to allow Bengal groundwater to significantly influence the marine Os isotopic composition, if likely fresh groundwater discharge rates to the Bay of Bengal are assumed. However, if Bengal groundwater Os concentrations are typical, the global Os groundwater flux would be expected to be around 180 kg/year, making it the second largest input of Os to the ocean after the river flux. Including this flux in the current Os marine budget, and assuming that this and other fluxes have remained constant with time, would decrease the calculated residence time of Os in the ocean by about 30%.     

Iron colloids/organic matter associated transport of major and trace elements in small boreal rivers and their estuaries (NW Russia)

The chemical status of major and trace elements (TE) in various boreal small rivers and watershed has been investigated along a 1500-km transect of NW Russia. Samples were filtered in the field through a progressively decreasing pore size (5, 0.8 and 0.22 μm; 100, 10, and 1 kD) using a frontal filtration technique. All major and trace elements and organic carbon (OC) were measured in filtrates and ultrafiltrates. Most rivers exhibit high concentration of dissolved iron (0.2–4 mg/l), OC (10–30 mg/l) and significant amounts of trace elements usually considered as immobile in weathering processes (Ti, Zr, Th, Al, Ga, Y, REE, V, Pb). In (ultra)filtrates, Fe and OC are poorly correlated: iron concentration gradually decreases upon filtration from 5 μm to 1 kD whereas the major part of OC is concentrated in the <1–10 kD fraction. This reveals the presence of two pools of colloids composed of organic-rich and Fe-rich particles. According to their behavior during filtration and association with these two types of colloids, three groups of elements can be distinguished: (i) species that are not affected by ultrafiltration and are present in the form of true dissolved inorganic species (Ca, Mg, Li, Na, K, Sr, Ba, Rb, Cs, Si, B, As, Sb, Mo) or weak organic complexes (Ca, Mg, Sr, Ba), (ii) elements present in the fraction smaller than 1–10 kD prone to form inorganic or organic complexes (Mn, Co, Ni, Zn, Cu, Cd, and, for some rivers, Pb, Cr, Y, HREE, U), and (iii) elements strongly associated with colloidal iron in all ultrafiltrates (P, Al, Ga, REE, Pb, V, Cr, W, Ti, Ge, Zr, Th, U). Based on size fractionation results and taking into account the nominal pore size for membranes, an estimation of the effective surface area of Fe colloids was performed. Although the total amount of available surface sites on iron colloids (i.e., 1–10 μM) is enough to accommodate the nanomolar concentrations of dissolved trace elements, very poor correlation between TE and surface sites concentrations was observed in filtrates and ultrafiltrates. This strongly suggests a preferential transport of TE as coprecipitates with iron oxy(hydr)oxides. These colloids can be formed on redox boundaries by precipitation of Fe(III) from inflowing Fe(II)/TE-rich anoxic ground waters when they meet well-oxygenated surface waters. Dissolved organic matter stabilizes these colloids and prevents their aggregation and coagulation. Estuarine behavior of several trace elements was studied for two small iron- and organic-rich rivers. While Si, Sr, Ba, Rb, and Cs show a clear conservative behavior during mixing of freshwaters with the White sea, Al, Pb and REE are scavenged with iron during coagulation of Fe hydroxide colloids.     

Size Fractionation of Trace Elements in a Seasonally Stratified Boreal Lake: Control of Organic Matter and Iron Colloids

The colloidal distribution and size fractionation of organic carbon and trace elements were studied in a seasonally stratified, organic-rich boreal lake, Lake Maselga, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 5?years in winter (glacial) and summer periods and during the spring and autumn overturn. A newly developed in situ dialysis technique (1, 10, and 50?kDa) and traditional frontal filtration and ultrafiltration (20, 10, 5, 0.22, and 0.025???m) were used to assess element concentrations at different depths. No significant changes in element concentrations occurred during filtration through sub-colloidal pore-size membranes (20?C0.22???m), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025?C0.22???m) were found to be the main carriers of poorly soluble elements (Fe, Al, Ti, Zr, REEs, Th, and U) during the summer and winter stratification. There was also a clear change in the vertical pattern of the percentage of colloidal Al, Ti, V, Cr, Fe, and Ni during different seasons, and the greatest proportion of colloidal forms was observed during the spring and autumn overturn. This pattern is most likely linked to the dominance of soil (allochthonous) organic carbon, which complexes with trace metals during these periods. During the summer seasons, autochthonous production of small exometabolites or photodegradation increases the concentration of the low-molecular weight fractions (<1?kDa) that dominate the speciation of divalent heavy metals in surface horizons. The colloidal status of As (30?C60%), which was documented in different seasons along the full depth of the water column, is most likely linked to the presence of organic complexes. The overall results of this study suggest that changes in the colloidal speciation of trace elements with depth in different seasons depend on changes in the redox conditions, the input of soil OM, the biodegradation of plankton biomass releasing dissolved organic matter in the bottom horizons, and in upward diffusion from the sediments.     

Present weathering rates in a humid tropical watershed: Nsimi, South Cameroon

The study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored.In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (<3 pmol/L, ICP-MS detection limits). Strongly insoluble secondary thorianite (ThO₂) and primary zircon (ZrSiO₄) crystals control their mobilization, respectively. This finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith.Chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed. Biogenic recycling significantly influences the low-Cl input fluxes. Sodium is a good tracer of chemical weathering in the watershed. The sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate. Even though low (2.8 mm/kyr), the chemical weathering rate predominates over the mechanical weathering rate (1.9 mm/kyr). Compared to the Rio Icacos watershed, the most studied tropical site, the chemical weathering fluxes of silica and sodium in the Mengong are 16 and 40 times lower, respectively. This is not only related to the protective role of the regolith, thick in both cases, but also to differences in the hydrological functioning. This is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level.     

The groundwater geochemistry of the Bengal Basin: Weathering, chemsorption, and trace metal flux to the oceans

Sixty-eight groundwater samples from the Ganges-Brahmaputra floodplain in the Bengal Basin were analyzed to assess the groundwater geochemistry, the subsurface hydrology, the buffering effects of sediments on trace metal concentrations and their isotopic compositions, and the magnitude of the subsurface trace element flux to the Bay of Bengal and to the global ocean. Samples obtained from depths of 10 to 350 m were measured for major and trace elements, dissolved gas, and tritium. On the basis of the ³He/³H ages, the groundwater at depth (30-150 m) appears to be continually replenished, indicating that this recharge of groundwater to depth must ultimately be balanced by a significant quantity of submarine discharge into the Bay of Bengal. Using the ³He/³H groundwater age-depth relationship to calculate a recharge rate of 60 ± 20 cm/yr, we estimate a subsurface discharge into the Bay of Bengal of 1.5 ± 0.5 × 10¹¹ m³/yr, or 15% of the surface Ganges-Brahmaputra river (GBR) flux. Several trace elements, especially Sr and Ba, display elevated concentrations averaging 7 to 9 times the surface GBR water values. The submarine groundwater fluxes of Sr and Ba to the oceans are 8.2 ± 2 × 10⁸ and 1.5 ± 0.3 × 10⁸ mol/yr, or 3.3 and 1.2%, respectively, of the world total, or equal to the surface GBR Sr and Ba estimated fluxes. Our groundwater flux for Ba agrees with the estimate of Moore (1997) (3 × 10⁸-3 × 10⁹ mol/yr), on the basis of measured Ba and Ra excesses in the Bay of Bengal. Other trace metals, such as U and Mo, are at low but measurable levels and are not major contributors to the global flux in this river system. A comparison of the Sr and Ba concentrations, plus ⁸⁷Sr/⁸⁶Sr ratios in groundwater to the oxalate extractable fractions of a coastal sediment core, suggests that weathering of carbonates and minor silicates, coupled with cation exchange plus adsorption and desorption reactions, controls the trace element concentrations and ⁸⁷Sr/⁸⁶Sr isotopic compositions in both the groundwater and river water. Our data also imply that other coastal floodplains (e.g., the Mekong and the Irrawaddy rivers) that have high precipitation rates and rapid accumulation of immature sediments are likely to make significant contributions to the global oceanic trace metal budgets and have an impact on the Sr isotopic evolution in seawater.     

Association of calcium with colloidal particles and speciation of calcium in the Kalix and Amazon rivers

A considerable amount of colloidally bound Ca has been detected in water samples from Amazonian rivers and the Kalix River, a sub-arctic boreal river. Fractionation experiments using several analytical techniques and processing tools were conducted in order to elucidate the matter. Results show that on average 84% of the total Ca concentration is present as free Ca. Particulate, colloidal and complexed Ca constitute the remaining 16%, of which the colloidal fraction is significant. Ultrafiltration experiments show that the colloidal fraction in the sampled Amazonian rivers and the Kalix River range between 1% and 25%.In both the Amazonian and the Kalix rivers the technique of cross-flow ultrafiltration was used to isolate particles and colloids. The difference in concentration measured with ICP-AES and a Ca ion-selective electrode in identical samples was used to define the free Ca concentration and thus indirectly the magnitude of the particulate, colloidal and complexed fractions. Results from the Kalix and Amazonian rivers are in excellent agreement. Furthermore, the results show that the colloidal concentrations of Ca can be greatly overestimated (up to 227%) when conventional analysis and calculation of ultrafiltration data is used due to retention of free Ca ions during the ultrafiltration process. Calculation methods for colloidal matter are presented in this work, using complementary data from ISE analysis.In the Kalix River temporal changes in the fractionation of Ca were studied before, during and after a spring-flood event. Changes in the size distribution of colloidally associated Ca was studied using FlFFF (Flow Field-Flow Fractionation) coupled on-line to a HR ICP-MS. The FlFFF-HR ICP-MS fractograms clearly show the colloidal component of Ca, supporting the ultrafiltration findings. During winter conditions the size distribution of colloidally associated Ca has a concentration maximum at ∼5 to 10 nm in diameter, shifting to smaller sizes (<5 nm) during and after the spring flood. This shift in size distribution follows a change in the river during this period from ironoxyhydroxy colloids being the most important colloidal carrier phase to humic substances during and after the spring flood.WHAM and NICA-Donnan models were used to calculate the amount of colloidally bound Ca. The results similar for both models, show that on average 16% of the Ca may be associated to a colloidal phase, which is in broad agreement with the measurements.     

长江源多年冻土区典型小流域水文过程特征研究

基于长江源风火山多年冻土区典型小流域2004-2007年的观测资料,运用回归和统计分析方法,探讨了气温、地温、降水、土壤水分与径流的响应关系.结果表明:5-9月为研究流域的水文过程活跃期,该时段的径流量占全年总径流量的85%;径流过程中存在春汛和夏汛,夏汛洪峰值明显高于春汛.径流过程中不同时段具有不同的影响主导因子,春汛期主要是65 cm以上的土壤温度和水分起主导作用;夏季枯水期40 cm以上土壤水分与径流是负相关,深层地温和土壤水分起主导作用;夏汛期气温、降水起主导作用;秋季枯水期的影响因子依次为土壤温度、水分及气温.多年冻土区降水大部分冻结在土壤中或者用来补充土壤水分的亏缺,不能直接产流,只有在春汛和夏汛期间与径流保持一定的相关关系.     

How important is it to integrate riverine suspended sediment chemical composition with depth? Clues from Amazon River depth-profiles

The vertical variability in mineralogical, chemical and isotopic compositions observed in large river suspended sediments calls for a depth-integration of this variability to accurately determine riverine geochemical fluxes. In this paper, we present a method to determine depth-integrated chemical particulate fluxes of large rivers, based on river sampling along depth-profiles, and applied to the Amazon Basin lowland tributaries. The suspended particulate matter (SPM) concentration data from depth-profiles is modeled for a number of individual grain size fractions using the Rouse model, which allows to predict the grain size distribution of suspended sediment throughout the whole river cross-section. Then, using (1) the relationship between grain size distribution and the Al/Si ratio (2) relationships between the Al/Si ratio and the chemical concentrations, the chemical composition of river sediment is predicted throughout the river cross-section, and integrated to yield the depth-integrated chemical particulate flux for a number of chemical elements (e.g. Si, Al, Fe, Na, REEs, …). For elements such as Al, Fe, REEs, Th, the depth-integrated flux is around twice as high as the one calculated from river surface sample characteristics. For Na and Si, the depth-integrated flux is three times higher than the “surface” estimate, due to the enrichment of albite and quartz at the bottom of the river. Depth-integrated ⁸⁷Sr/⁸⁶Sr composition of suspended sediment, also predictable using this method, differs by more than 10⁻³ from the surface sample composition.Finally, potential implications of depth-integrated estimates of Amazon sediment chemistry are explored. Depth-integration of particulate ⁸⁷Sr/⁸⁶Sr isotopic ratios is necessary for a reliable use of Sr isotopes as a provenance tracer. The concept of steady-state weathering of a large river basin is revisited using depth-integrated sediment composition. This analysis shows that, in the Amazon Basin river, the previously observed discrepancy between (1) weathering intensities of channel surface sediment and (2) silicate-derived dissolved fluxes is only slightly accounted for by the vertical variability of suspended sediment weathering intensities. This observation confirms that most large rivers basins are not eroding at steady-state.     

Geochemistry of organic-rich river waters in Amazonia: Insights on weathering processes of intertropical cratonic terrain

In this study, eight organic-rich rivers that flow through the Brazilian craton in the southwestern Amazon rainforest are investigated. This investigation is the first of its type in this area and focuses on the effects of lithology, long-term weathering, thick soils, forest cover and hydrological period on the dissolved load compositions in rivers draining cratonic terrain. The major dissolved ion concentrations, alkalinity (TAlk), SiO₂, trace element concentrations, and Sr isotope contents in the water were determined between April 2009 and January 2010. In addition, the isotopic values of oxygen and hydrogen were determined between 2011 and 2013. Overall, the river water is highly dilute and dominated by the major dissolved elements TAlk, SiO₂ and K⁺ and the major dissolved trace elements Al, Fe, Ba, Mn, P, Zn and Sr, which exhibit large temporal and spatial variability and are closely correlated with the silicatic bedrock and hydrology. Additionally, rainwater and recycled water vapor and the size of the basin contribute to the geochemistry of the waters. The total weathering flux estimated from our results is 2–4 t km⁻².yr⁻¹, which is one of the lowest fluxes in the world. The CO₂ consumption rate is approximately 21–61 10³ mol km⁻² yr⁻¹, which is higher than expected given the stability of the felsic to basic igneous and metamorphic to siliciclastic basement rocks and the thick tropical soil cover. Thus, weathering of the cratonic terrain under intertropical humid conditions is still an important consumer of CO₂.     

Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. ⁸⁷Sr/⁸⁶Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm⁻² ka⁻¹. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater ⁸⁷Sr/⁸⁶Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.     

Effect of source integration on the geochemical fluxes from springs

Geochemical fluxes from watersheds are typically defined using mass-balance methods that essentially lump all weathering processes operative in a watershed into a single flux of solute mass measured in streamflow at the watershed outlet. However, it is important that we understand how weathering processes in different hydrological zones of a watershed (i.e., surface, unsaturated, and saturated zones) contribute to the total geochemical flux from the watershed. This capability will improve understanding of how geochemical fluxes from these different zones may change in response to climate change. Here, the geochemical flux from weathering processes occurring solely in the saturated zone is investigated. This task, however, remains exceedingly difficult due to the sparsity of subsurface sampling points, especially in large, remote, and/or undeveloped watersheds. In such cases, springflow is often assumed to be a proxy for groundwater (defined as water residing in fully saturated geologic formations). However, springflow generation may integrate different sources of water including, but not limited to, groundwater. The authors’ hypothesis is that long-term estimates of geochemical fluxes from groundwater using springflow proxies will be too large due to the integrative nature of springflow generation. Two conceptual models of springflow generation are tested using endmember mixing analyses (EMMA) on observations of spring chemistries and stable isotopic compositions in a large alpine watershed in the San Juan Mountains of southwestern Colorado. In the “total springflow” conceptual model, springflow is assumed to be 100% groundwater. In the “fractional springflow” conceptual model, springflow is assumed to be an integration of different sources of water (e.g., groundwater, unsaturated flow, preferential flow in the soil, etc.) and groundwater is only a fractional component. The results indicate that groundwater contributions in springflow range from 2% to 100% overall and no springs are consistently composed of 100% groundwater; providing support for the fractional springflow conceptual model. Groundwater contributions are not strongly correlated with elevation, spring contributing area, spring discharge, or seasonality. This variability has a profound effect on long-term geochemical fluxes. The geochemical fluxes for total springflow overestimate long-term solute release by 22–48% as compared to fractional springflow. These findings illustrate that springflow generation, like streamflow generation, integrates many different sources of water reflecting solute concentrations obtained along many different geochemical weathering pathways. These data suggest that springs are not always ideal proxies for groundwater. Springs may be integrating very distinct portions of the groundwater flow field and these groundwater contributions may become mixed at the spring emergence with much younger sources of water that have never resided in the groundwater system.     

Evaluating natural and anthropogenic trace element inputs along an alpine to urban gradient in the Provo River,Utah, USA

Numerous natural and anthropogenic processes in a watershed produce the geochemical composition of a river, which can be altered over time by snowmelt and rainfall events and by built infrastructure (i.e., dams and diversions). Trace element concentrations coupled with isotopic ratios offer valuable insights to disentangle the effects of these processes on water quality. In this study, we measured a suite of 40+ trace and major elements (including As, Cd, Ce, Cr, Cs, Fe, La, Li, Mo, Pb, Rb, Sb, Se, Sr, Ti, Tl, U, and Zn), Sr isotopes (⁸⁷Sr/⁸⁶Sr), and stable isotopes of H and O (δD and δ¹⁸O) to investigate natural and anthropogenic processes impacting the Provo River in northern Utah, USA. The river starts as a pristine mountain stream and passes through agricultural and urban areas, with two major reservoirs and several major diversions to and from the river. We sampled the entire 120 km length of the Provo River at 13 locations from the Uinta Mountains to Utah Valley, as well as two important tributaries, across the range of hydrologic conditions from low flow to snowmelt runoff during the 2013 water year. We also sampled the furthest downstream site in the Utah Valley urban area during a major flood event. Trace element concentrations indicate that a variety of factors potentially influence Provo River chemistry, including inputs from weathering of carbonate/siliciclastic rocks (Sr) and black shales (Se and U), geothermal groundwater (As, Cs, Li, and Rb), soil erosion during snowmelt runoff (Ce, Cr, Fe, La, Pb, and Ti), legacy mining operations (Mo, Sb, and Tl), and urban runoff (Cr, Pb, and Zn). Although specific elements overlap between different groups, the combination of different elements together with isotopic measurements and streamflow observations may act as diagnostic tools to identify sources. ⁸⁷Sr/⁸⁶Sr ratios indicate a strong influence of siliciclastic bedrock in the headwaters with values exceeding 0.714 and carbonate bedrock in the lower reaches of the river with values approaching 0.709. δD and δ¹⁸O changed little throughout the year in the Provo River, suggesting that the river is primarily fed by snowmelt during spring runoff and snowmelt-fed groundwater during baseflow. Based on nonmetric multidimensional scaling (NMS) water chemistry was unique across the upper, middle, and lower portions of the river, with high temporal variability above the first reservoir but minimal temporal variability below the reservoir. Thus, the results show that dams alter water chemistry by allowing for settling of particle-associated elements and also by homogenizing inflows throughout the year to minimize dilution during snowmelt runoff. Taken together, trace element concentrations and isotopic measurements can be used to evaluate the complex geochemical patterns of rivers and their variability in space and time. These measurements are critical for identifying natural and anthropogenic impacts on river systems.     

Temporal variations of colloidal carrier phases and associated trace elements in a boreal river

Elemental size distributions, from truly dissolved through colloidal to particulate, have been studied in a subarctic boreal river. The measurements, carried out during 2002, ranged from winter to summer conditions, including an intense spring flood event. Results are reported for a total of 42 elements. Size distributions were characterised using a combination of cross-flow (ultra)filtration (CFF), flow field-flow fractionation (FlFFF), and diffusive gradients in thin-films (DGT). The three techniques showed similar trends, but quantitative comparisons reveal some important differences that warrant further investigation.Previous work has identified two colloidal carrier phases in fresh waters, dominated by iron and carbon, respectively. The majority of the elements studied are associated with one or both of these colloidal carrier phases. The exceptions are the alkali metals and several anions that are only very weakly associated with colloidal material, and which therefore occur mainly as truly dissolved material (<1 kDa in molecular weight). We discuss the likely origin for the two colloidal carrier phases and consider how associated trace elements fit into the geochemical framework. The relative affinities of the elements for iron and carbon colloidal carrier phases are related to their chemistries, and are compared with earlier data from the Delsjö Creek in southern Sweden.Elemental colloidal concentrations show strong seasonal variations related to changes in the colloidal carrier phase(s) with which they associate. In particular, many elements show a strong spring maximum in colloidal concentrations associated with the strong maximum in colloidal carbon concentration during the spring flood.     

Multi-element geochemistry of sediments from the Pearl River system,China

Sediment samples were taken along the West, North, and East rivers of the Pearl River system at 28 locations in 1998, and a total of 49 elements were determined by ICP–AES, ICP–MS and INAA. The probability features of the datasets were studied, and the average concentrations of these elements in sediments of the three rivers were calculated. Significant differences in element concentrations among the three rivers were observed and the results were confirmed by statistical tests including analysis of variance (ANOVA), Kruskal–Wallis test, and t-test. Spatial distribution maps of element concentrations were produced using a geographical information system (GIS). The immobile trace elements (such as Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) are enriched in the West River where limestone dominates the rock types in the watershed. Because of the strong weathering, immobile trace elements are enriched and reside in secondary minerals of the weathering products. All three rivers have high concentrations of rare earth elements (REEs) because of strong weathering, but relatively higher concentrations of REEs are observed in sediments of the East River where granite dominates the rock type. Granite contains high concentrations of REEs and the sediments have inherited this feature from their bedrock. Alkaline element (Li, Na, K, Rb, and Cs) concentrations are elevated in sediments of the East River, these may reside in granitic primary minerals. Relatively high concentrations of alkaline earth elements (Mg, Ca, Sr, and Ba) are observed in the West River, inherited from the limestone bedrock. High Pb and Bi concentrations are found in the North River and are caused by Pb mineralization and the discharge of a smelter in the upper reaches of the river. However, statistical tests did not indicate a significant difference between Pb concentrations in the North River and the other two rivers, which suggests that statistical results should be carefully used and explained.     

Suspended basaltic glass–seawater interactions

The objective of this study was to quantify by experiments the initial seawater–suspended basaltic glass interactions following the 1996 outburst flood from the Vatnajökull glacier, Iceland. The altered basaltic glass dissolved in seawater as recorded by the Si release from the glass. The dissolved concentrations of Na, Ca, Si, Ba, Cd, Co, Cu, Hg, Mn, Ni and total dissolved inorganic N increased with time but the concentrations of Mg, K, S, Sr, Fe, Pb and Zn decreased. Calculated 1 to 10 day fluxes for Si range from 38,000 tons/day to 70,000 tons/10 days. The fluxes for other major elements are more uncertain, but the positive flux (release from suspended matter to seawater) of Ca and Na, and negative flux of Mg, K and S are greater than the Si flux.     

Wash-out processes of evaporitic sulfate salts in the Tinto river: Hydrogeochemical evolution and environmental impact

This work deals with the wash-out processes of evaporitic salts in the Tinto river in October 2005, with the arrival of the first rainfall after the summer. In order to monitor water levels and electrical conductivity, a datalogger was set up in the river, while sampling was performed by a portable autosampler. Thirty-two samples were selected for analysis for a wide range of elements by ICP-AES. Three different flood events, with a maximum discharge of 8.1 m³/s, were monitored. River waters suffered from a dilution effect at the beginning of the first event, recording a concentration decrease of most elements, just before the wash-out of soluble salts precipitated during the summer took place. Wash-out processes provoked a sharp increase in most element concentrations coinciding with an intense decrease in Na and Sr. After the first event, there was strong enrichment of As, and to a lesser extent in Fe, Cr and Pb, due probably to the redissolution/transformation of Fe oxyhydroxysulfates. During the third event, evaporitic sulfate salts were depleted from riverbanks ending wash-out processes, and a decrease in most element concentrations was observed. Barium exhibited different behaviour to the rest of elements owing to the solubility control exerted by barite. Lead also showed a different pattern throughout the study period, its concentration decreasing due probably to its great affinity for coprecipitate on jarosite-group minerals, and increasing when schwertmannite precipitation is thermodynamically favoured. In October 2005, the Tinto river carried around 8100 t SO₄, 1300 t Fe, 400 t Al, 100 t Zn and Cu, etc., highlighting the importance of wash-out processes of soluble salts in the Ría de Huelva ecosystem and metal fluxes into the Atlantic Ocean.     

Variations of U and Sr isotope ratios in Alsace and Luxembourg rain waters: origin and hydrogeochemical implications

Major and trace element concentrations, as well as Sr and U isotope ratios, were measured in rainwater samples collected in three different locations in Alsace (East of France) and Luxembourg: a mid-altitude mountain site (Aubure Environment HydroGeochemical Observatory), an urban site (Strasbourg) and a peri-urban site located in an area of well developed industrial activity (Esch-sur-Alzette in Luxembourg). Results highlight the quite high spatial and temporal variability of the chemical and isotopic characteristics of rainwater at the regional scale. They also suggest a quite systematic contribution of a local component in the chemical composition of rainwater. In urban and peri-urban sites, the local component is certainly linked to human activities, as it is well illustrated in this study with the Esch-sur-Alzette samples. On the other hand, for the Aubure site, i.e. a small forested watershed of mid-altitude mountain, data presented in this study demonstrate the influence of the vegetation on the chemical composition of rainwater for alkali and calc-alkali elements, as well as for the Sr isotope ratios. Such a result questions the reliability of the method classically used to estimate the rainwater contribution on the river chemical budget. In addition, data of the present study confirm the very low content of uranium in rainwater and demonstrate, especially through the U isotope analysis of Aubure rainwater, the negligible effect of rainwater on U budget of river waters. This work thus outlines the property of U to be a geochemical tracer specific of weathering fluxes carried by rivers. To cite this article: F. Chabaux et al., C. R. Geoscience 337 (2005).